This invention relates to aircraft engines, and, more particularly, to the design of a turbine rotor.
A jet engine draws air into the front of the engine, compresses the air with a rotating compressor that is mounted on a rotating shaft, and mixes fuel with the compressed air. The mixture of fuel and air is burned in a combustor, and the hot exhaust gases are passed through a turbine. The turbine is supported on the same shaft as the compressor, so that the turbine provides the power to operate the compressor.
In the axial flow jet engine, the turbine includes a set of stationary turbine vanes that deflect the flow of hot exhaust gases, and a turbine rotor having turbine blades mounted on the rim or periphery of a turbine disk. The center of the turbine disk is supported on the shaft. Hot exhaust gases pass through the vanes and are deflected slightly. The deflected gases impinge upon the turbine blades and force them sideways, causing the turbine disk and thence the shaft to turn. The preceding description of a jet engine is intended to be conceptual in nature, and it will be appreciated that there are typically numerous compressor and turbine stages mounted on the central shaft, and other complex structure.
The turbine blades are typically made of metallic superalloys having superior strength and creep properties at temperatures of 1800-2100 F. The turbine disk may be made of the same material, or a material having superior strength and fatigue resistance properties at lower temperatures, inasmuch as the turbine disk does not experience as high a temperature as do the turbine blades.
Even with the careful selection of material properties for the turbine disk, it is observed that fatigue cracks can form in the rim of the turbine disk at locations between the turbine blades. These cracks can then propagate into the body of the turbine disk, unless stopped, and eventually lead to failure of the turbine disk. One approach to reducing the incidence of fatigue cracks in the rim of the turbine disk is to make short radial slots in the rim of the turbine disk between the turbine blades. A hole is drilled at the bottom of each of the slots to increase the radius of curvature of the slot, thereby reducing the stress concentration at the bottom of the slot to prevent the slot from propagating as a crack. Thus, uncontrolled formation and propagation of fatigue cracks in the rim of the turbine disk is avoided by artificial placement of controlled slots to relax the stresses that are produced in the rim of the turbine disk.
This slotting approach is operable and has been widely used. However, it has the shortcoming that the rim of the turbine disk is weakened and the centrifugal loading of the remainder of the disk hub is increased by the presence of the slots. There is therefore a need for some improved approach to reducing the incidence of fatigue cracks in the rim of the turbine disk.